Nuclear magnetic resonance (NMR) spectroscopy can be used as a tool for determining the chemical structure and/or geometry of a molecule in a sample. In many samples, however, resonance frequencies of different nuclei fully or partially overlap, which makes chemical identification of molecule(s) in a sample difficult or impossible.
Nuclear singlet states are non-magnetic states of nuclear spin pairs that may exhibit extraordinarily long lifetimes. The long lifetime suggests that nuclear singlet states can be used to the study a variety of processes, e.g., slow motions, chemical exchange and transport of hyperpolarized spin order. Moreover, singlet states can also be used as a quantum filter to target molecules hidden in a complicated spectrum, as demonstrated with a pulse sequence referred to as “SUCCESS.” The rate of singlet decay has also been shown to carry information on the locations of neighboring magnetic nuclei.
Nuclear spin singlet states in proton pairs can exhibit lifetimes much longer than the spin lattice relaxation time, T1. Such states exist naturally when nuclear spins are strongly coupled relative to their resonance frequency differences, i.e., J>>Δν. However, due to the differences in singlet and triplet symmetries, it is not possible to transfer magnetization to the singlet state by directly driving a radiofrequency transition. Tayler and Levitt demonstrated that such a transfer can instead be achieved using a series of π-pulse trains in which the pulse timing is synchronized to the J-coupling strength between nuclei (Tayler et al., Physical Chemistry Chemical Physics 13, 5556(2011)). This “M2S” sequence takes advantage of the small amount of mixing between singlet and triplet states that is present whenever Δν>0. Feng and Warren also showed that the sequence can create singlet states in certain heteronuclear systems even when the nuclear spins are identical (Fen et al., Nature Physics 8, 831 (2012)). These sequences hold promise for creating hyperpolarized singlet states without the need for a chemical reaction or continuous spin-locking (Warren et al., Science 323, 1711 (2009)). However, in both implementations, the transfer to singlet state only occurs during the final third of the sequence time, and before this stage the total magnetization is present in states subject to conventional spin-lattice relaxation.